DIATOMS 181 
larger new cell (“‘rejuvenescence”), They generate by 
the escape and union of the protoplasms of two contigu- 
ous cells whose half-cells have separated, resulting in the 
formation of one or two new and usually much larger 
cells. Small biciliate isogametes have been doubtfully 
reported in some marine diatoms. 
260. There are two general kinds (orders) of Diatoms, 
namely, the Round Diatoms (Zupodiscales) with the cells 
mostly round in end view, and the Flat Diatoms 
(Naviculales) with the filaments flattened in end view. 
261. The Round Diatoms are mostly 
marine and fossil. The ends of the cells J-LUVJ 
are usually marked radially with lines or ay 
rows of dots, as in Melosira, Coscinodiscus, fre. 73.—A 
- Actinodiscus, ete. Some Round Diatoms Rend Distom: 
form long filaments (Melosira). 
262. The Flat Diatoms occur abundantly as fresh- 
water, marine, and fossil plants. The ends of the cells 
(transection of the flat filament) are often 
| marked transversely or pinnately by dots or 
i 
lines. In many of our most common Flat 
Diatoms (e.g. Naviculaceae) there is a me- 
dian longitudinal slit (“‘raphe’’) in the end 
ine Paeiome: wall, which probably has to do with the mo- 
Bacillaria. ‘tility exhibited by these plants (Par. 174). 
263. Origin of Zygophyceae. It may be assumed that 
the plants of this phylum have been derived from other 
filamentous plants, and that the adhesion of cell to cell, 
and the consequent formation of a multicellular plant 
body, had become a well established habit long before 
the peculiarties arose which set them off as Zygophy- 
ceae. We must search among the Confervoideae of the 
preceding phylum for the ancestral types from which the 
Conjugate Algae may have descended. Such plants as 
SS 
